1. Field of the Invention
The present invention relates to a nitride semiconductor device for use in a laser diode or light-emitting diode that emits blue light and to a method of fabricating such a nitride semiconductor device. More particularly, the present invention relates to a nitride semiconductor device having a nitride semiconductor substrate and to a method of fabricating such a nitride semiconductor device.
2. Description of the Prior Art
Expectations have been running high for and various applications have been attempted with III–V nitride semiconductors composed of a group Ill element such as Al, Ga, or In and a group V element N (hereinafter such a semiconductor will be referred to as a “GaN-based semiconductor”) as light-emitting devices and power devices for their desirable band structure and chemical stability. For example, many attempts have been made to lay a layer of a GaN-based semiconductor on a sapphire substrate to produce a nitride semiconductor device that emits blue laser. In general, in AlGaInAs- or AlGaInP-based nitride semiconductor devices, cavities, which are essential for laser oscillation, are produced by the use of cleavage planes.
However, in a case where a GaN-based semiconductor layer is laid on a sapphire substrate, since sapphire does not cleave easily, the end surfaces of the produced chip have surface irregularities as large as 4 to 10 nm on average, making it difficult to obtain a satisfactory cavity. Furthermore, in a case where a nitride semiconductor device is formed by laying a GaN-based semiconductor layer on a sapphire substrate, since, in general, the angle at which sapphire cleaves is 30° apart from the angle at which the GaN-based semiconductor layer laid on the substrate cleaves, it is difficult to reduce the surface irregularities on the end surfaces irrespective of along which of the substrate's and the upper layer's cleavage planes the chip is diced apart.
For these reasons, much attention has been paid to using, as a substrate on which to lay a GaN-based semiconductor layer, a GaN-based substrate that cleaves easily and that cleaves in the same direction as the GaN-based semiconductor layer laid on its surface and producing the end surfaces by cleavage. Here, a GaN-based substrate denotes a substrate formed out of a GaN-based semiconductor. When a GaN-based substrate is used, the GaN-based semiconductor layer and the GaN-based substrate cleave in the same direction, and therefore the end surfaces are expected to be flat. Moreover, when a GaN-based substrate is used, good lattice matching is achieved between the GaN-based substrate and the GaN-based semiconductor layer laid on it, and no difference exists between their thermal expansion coefficients. This helps reduce the strain on and hence defects of the nitride semiconductor device, and is thus expected to extend the useful life of the nitride semiconductor device.
An example of a nitride semiconductor device in which, as described above, a GaN-based semiconductor layer is laid on a GaN-based substrate and then the end surfaces of the cavity are produced by cleavage is disclosed, for example, in Japanese Patent Application Laid-Open No. H11-4048.
However, concerning the abovementioned example of a nitride semiconductor device using a GaN-based substrate disclosed in Japanese Patent Application Laid-Open No. H11-4048, there is given no detailed description about how the end surfaces of the cavity are produced or how the chip is diced apart. This has led the inventors of the present invention to try in various ways how a wafer using a GaN-based substrate cleaves, only to find that, in practice, it is difficult to dice such a wafer with a constant, uniform cavity length and at a satisfactory yield rate.
For example, in the dicing process of a wafer 130, as shown in FIG. 13, having stripe-shaped optical waveguides 131 and having cleavage guide grooves 132 formed at an edge so as to run in the direction of cleavage, ideally, cavities are produced as a result of the wafer 130 being cleaved along cleaving lines, like the one designated as 133, running in the same direction as the cleavage guide grooves 132. Cleaving along such cleaving lines 133 permits the stripe-shaped optical waveguides 131 to be split with flat surfaces. This makes it possible to produce nitride semiconductor devices at a high yield rate. In reality, however, many dicing lines meander, like the one designated as 134, or run at 60°, like the one designated as 135, relative to the desired dicing direction.
One cause of the formation of such unintended dicing lines as those designated as 134 and 135 is that, even when cleavage guide grooves 132 are formed in the <11-21> direction (of which a description will be given later) in which cleavage occurs, in a GaN-based substrate, which has a hexagonal crystal structure, directions that are at 60° relative to that direction are equally valid cleavage directions, and therefore cleavage occurs as easily also along lines that are at 60° relative to the desired dicing direction. If such unintended cleavage occurs only once, a dicing line like the one designated as 135 in FIG. 13 results; if such unintended cleavage occurs continuously, a dicing line like the one designated as 134 results. Another cause is that, compared with a sapphire substrate, a GaN-based substrate is so brittle as to make cleavage in inclined directions as described above more likely, causing, in the worst case, the devices to be broken to pieces.